The present invention is directed to semiconductor devices and, more particularly, to a trench MOSFET with low gate charge having improved switching speed characteristics.
In a typical trench MOSFET, such as that schematically depicted in FIG. 1, gate-to-drain capacitance varies with gate and drain voltage, having a high value during the on-state. As the drain voltage increases, the gate-to-drain capacitance decreases. Amplification of gate-to-drain capacitance by feedback, referred to as Miller capacitance, can result in severe switching loss and a marked reduction in the frequency response
FIG. 2 is a plot of gate charge, Qg, versus gate-source voltage, Vgs. The horizontal portion, which illustrates the effect of Miller capacitance, occurs at the turn-on voltage, where the rapidly falling drain forces the gate driver to supply additional charge to the gate-to-drain capacitance. Reducing or eliminating Miller capacitance, as provided by the present invention, would result in improved switching characteristics of the MOSFET.
The present invention is directed to a trench MOS-gated device having an upper surface and comprising a substrate that includes an upper layer comprising doped monocrystalline semiconductor material of a first conduction type. A gate trench in the upper layer has sidewalls and a floor lined with a first dielectric material and a centrally disposed core that comprises a second dielectric material that has a lateral surface and a top surface and extends upwardly from the first dielectric material on the trench floor. The remainder of the trench is substantially filled with a conductive material that encompasses and contacts the lateral surface and the top surface of the core of second dielectric material.
A doped well region of a second conduction type overlies a drain zone of the first conduction type in the upper layer, and a heavily doped source region of the first conduction type contiguous to the gate trench and a heavily doped body region of the second conduction type are disposed in the well region at the upper surface of the device. An interlevel dielectric layer disposed on the upper surface overlies the gate trench and the source region, and a metal layer in electrical contact with the source and body regions overlies the upper surface and the interlevel dielectric layer.